Gear pumps consist of two meshing gear wheels which are mounted on shafts, wherein generally one shaft is connected to a drive unit. The shaft which is not being driven by a drive unit is driven by torque transmission from the shaft being driven via the tooth flanks.
Often problems of wear occur at the tooth flanks through excessive contact pressure due to the torque transmission. Namely, on the one hand because the pressure load brought about by the pressure difference needs to be transferred via the tooth flanks from the shaft being driven to the driven shaft, and on the other hand because the friction needs to be overcome. Damages on the tooth flanks can occur through abrasion or wear (pittings, micro-weldings, abrasion), especially in the manufacturing of polymers in large polymerization installations or in compounding of plastics with large throughputs at very high backpressures and high temperatures, i.e. overall high torque.
In order to avoid these damages two shaft drives have already been used, for which the propulsion takes place with a single drive unit (motor), and then subsequently the force is distributed by a mechanical transfer box to the two gear pump shafts.
Furthermore, from the patent CH-659 290 a gear pump is known in which the two shafts are each driven with a drive unit. Each of the two gear wheels draws the necessary drive power from the associated drive unit. Only relatively minor power differences are transmitted between the two gear wheels.
From the EP-0 886 068 B1 a gear pump is known in which again two drive units are provided for individually driving the shafts, where the phase and angular velocity of the meshing gear wheels are coordinated in such a way that on the one hand a lifting off of tooth flanks of the meshing gear wheels and on the other hand an excessive excess torque on the tooth flanks of the meshing gear wheels are avoided.
It has been shown that in known gear pumps the wear, especially when conveying abrasive media, can be substantial.
In particular in extrusion applications of highly filled, abrasive polymer melts the problem of high tooth flank wear due to abrasion can occur and thus the problem of premature failure of the gear wheel shafts arises, since abrasive particles contained in the melt are ground between the tooth flanks. Thereby, damage and abrasion of the surface of the tooth flanks can be caused. Furthermore, the viscosity of the melt is increased due to loading with filler material and therewith the torque requirement of the entire pump or the required torque at the individual shafts rises, so that a possible exceedance of the allowed contact pressure at the tooth flanks again becomes the focus of attention.
It is therefore an object of the present invention to provide a method of controlling a gear pump with which an improvement is achieved with respect to a least one of the mentioned disadvantages.
This object is achieved by the method of the invention of controlling a gear pump which comprises determining a current position of the one gear wheel with respect to the current position of the other gear wheel, and continuously adjusting the current position of the one gear wheel with respect to the current position of the other gear wheel according to specified predefined operating conditions. Preferred embodiments as well as an application/arrangement according to the invention are disclosed hereinafter.
The present invention relates to a method of controlling a gear pump comprising two meshing gear wheels, wherein the two gear wheels are driven via respective shafts each by a drive unit. The invention is characterized in that a current position of the one gear wheel is determined with respect to the current position of the other gear wheel and that the current position of the one gear wheel is continuously adjusted with respect to the current position of the other gear wheel according to specified predefined operating conditions.
One embodiment is characterized in that the determination of the present position of the one gear wheel with respect to the present position of the other gear wheel is adjusted via a reference value, which is determined before the normal operation of the gear pump or during interruptions of the normal operation of the gear pump.
Further embodiments of the present invention are characterized in that the reference value lies in the middle between tooth flanks of a tooth space of a gear wheel, preferably in the middle between tooth flanks of a tooth space of a gear wheel.
Further embodiments of the present invention are characterized in that the reference value is determined in that,                the one gear wheel is driven by the other gear wheel with a predetermined torque,        a first angle difference is determined by calculating the difference between values measured with the rotary encoders/sensor units,        the other gear wheel is driven by the one gear wheel with a predetermined torque,        a second angle difference is determined by calculating the difference between values measured with the rotary encoders/sensor units,        a difference is determined between the first angle difference and the second angle difference, and        the reference value is specified within the determined difference.        
Thus a method for automatically calibrating the arrangement including a gear pump is provided. The system can perform this calibration both before the beginning of operation as well as during service interruptions without further action by the operator. With this method a possible wear of tooth flanks can also be detected, since then the difference between the first angle difference and the second angle difference also changes or increases. An excessive wear can then be simply detected when exceeding a threshold value.
Further embodiments of the present invention are characterized in that at least one of the following current values is monitored:                the first angle difference,        a difference between the first angle difference and the reference value,        the second angle difference,        a difference between the second angle difference and the reference value,        
and that in case of under- or overshooting of the at least one of the current values below a predefined value at least one of the following actions is executed:                an optical warning,        optical display,        acoustic warning,        change of operating conditions of the gear pump.        
Further embodiments of the present invention are characterized in that rotary encoders/sensor units are applied for determining the current position of the one gear wheel and the second gear wheel, wherein each rotary encoder/sensor unit is arranged centrally between the toothing of the respective gear wheel and a rotor of the respective drive unit.
A central arrangement of the rotary encoders/sensor units has the advantage that an existing torsion angle due to a non-ideal stiffness of the entire drive train has a reduced influence on the measurement error of the system. The measurement error is halved by the central arrangement.
Further embodiments of the present invention are characterized in that a predefined gear lash is adjusted between two meshing gear wheels.
Further embodiments of the present invention are characterized in that a leading flank, in the direction of rotation of the one gear wheel, of a tooth meshing into a tooth gap touches a lagging flank, in the direction of rotation of the other gear wheel, and that a leading flank, in the direction of rotation of the other gear wheel, of a tooth meshing into a tooth gap touches a lagging flank, in the direction of rotation of the one gear wheel.
This operation condition is also referred to a changeover of flanks since the flanks touching each another will change during the course of the extrusion process.
Further embodiments of the present invention are characterized in that the one gear wheel drives the other gear wheel with a predetermined torque, wherein the predetermined torque is greater than half of the total torque generated by both drives.
A precise torque setting can be achieved by appropriate control of the rotary speed or current positions of the gear wheels to each other. The tooth flanks thus transfer an arbitrary adjustable torque, however they never lift off from each other during operation if a defined flank sealing is always to be achieved.
Further embodiments of the present invention are characterized in that the rotary speed of the shafts driven by the drive units is synchronously adjusted in such a way that a pressure of the medium to be conveyed stays substantially constant on a discharge side of the gear pump.
The advantage associated with this is that no disturbing pulsation is present at the discharge side of the gear pump anymore, which is reflected in the quality of the extrudate.
Further embodiments of the present invention are characterized in that the pressure of the medium to be conveyed is measured on the discharge side of the gear pump and that the rotary speed is adjusted in dependence of the measured pressure of the medium.
Furthermore, an application of the method according to the present invention is provided for an arrangement including a gear pump, comprising a pump housing, two meshing gear wheels contained in the pump housing and two shafts which are operatively connected to the gear wheels and extend through the pump housing, wherein the two shafts are each operatively connected to respective drive units. According to the invention it is provided that a coupling unit for compensation of eccentricities between the drive unit and the respective shaft is arranged between each gear wheel and drive unit and that a rotary encoder/sensor unit is arranged between the center of the gear wheel and the center of the respective drive.
One embodiment of the present application is characterized in that the rotary encoder/sensor unit is located in an axial region which is defined by the center between the center of the gear wheel and the center of the drive plus a deviation on both sides of at most 10% of the distance between the center of the gear wheel and the center of the drive.
Further embodiments of the present application are characterized in that rotary encoders/sensor units are respectively arranged in the middle between the respective center of the gear wheel and the respective center of the drive.
Further embodiments of the present application are characterized in that the rotary encoders/sensor units feature a radial distance to the rotation axis of the respective shaft which is larger, preferably at least twice as large, as the outer radius of the gear wheels.
Further embodiments of the present application are characterized in that the rotary encoders/sensor units are either optical or magnetic rotary encoders/sensor units.
Further embodiments of the present application are characterized in that the rotary encoders/sensor units are arranged such that a connecting line which runs through the corresponding rotary encoder/sensor unit and extends perpendicularly from the shaft encloses together with a plane which extends centrally between the two rotation axes on a suction side an angle in the range of 35° to 55°, preferably 40° to 50°, preferably 45°.
Further embodiments of the present application are characterized in that each drive unit features a rotor and a stator, wherein the rotor is axially moveable with respect to the stator.
Further embodiments of the present application are characterized in that the drive unit features on the far side with respect to the gear pump a differential bearing unit which radially supports the rotor of the drive unit.
Further embodiments of the present application are characterized in that the rotor of the drive unit is connected to the respective shaft of the gear pump via a coupling unit.
Further embodiments of the present application are characterized in that the coupling unit is a membrane coupling.
Further embodiments of the present application are characterized in that a flange is arranged between the pump housing and the stator of the respective drive unit, wherein the flange features bores through which a cooling agent circulates for adjusting the temperature.
Further embodiments of the present application are characterized in that the drive units are connectable to the respective shaft of the gear pump from the far side with respect to the gear pump.
Further embodiments of the present application are characterized in that the connections between the drive units and the respective shafts of the gear pump are conical polygon connections.
Further embodiments of the present application are characterized in that the drive units are of the torque motor type.
Further embodiments of the arrangement according to the invention are characterized in that the one drive unit, the gear pump and the other drive unit are each contained in a temperature zone in which the temperatures are adjustable to specified values, wherein preferably isolation regions are present between neighboring temperature zones.